This invention relates generally to improvements in electron guns used in television receiver cathode ray tubes, and is more particularly addressed to improved means and method for adjusting certain operating parameters of electron guns to provide optimum beam spot size. The invention has applicability to television picture tube guns of many types and construction such as the unipotential, bipotential, the Einzel; guns having extended field main focus lenses, and guns designed for projection television. Such guns may be structured as a single gun, a triad of discrete guns, or the electrodes comprising the gun may be unitized and in delta or in-line configurations.
The type of electron gun in most common use for color cathode ray tubes is the unitized, in-line gun. Three electron beams are developed by cathodic thermionic emission. The beams are formed and shaped by a tandem succession of electrodes spaced along the central axis of the gun. These electrodes cause the beams to be selectively focused on groups of phosphors located on the faceplate of the cathode ray tube. The phosphors may comprise discrete triads consisting of red, green and blue dots, or more commonly, triads of red, green and blue stripes.
A prime objective in the design and manufacture of this and other types of electron guns is the projection of one or more small, symmetrical beam spots on the screen to provide maximum resolution of the television picture.
The structure and relationship of an electron gun and an associated picture tube, and the prior art means for supplying operating voltages to the combination, is indicated by FIG. 1. The primary components of a typical color picture tube 10 comprise an evacuated envelope including a neck 12, a funnel 14 and a faceplate 16. On the inner surface of the faceplate 16 are deposited a multiplicity of cathodoluminescent phosphor targets 18 comprising a pattern of groups of red-light-emitting, green-light-emitting, and blue-light-emitting dots or stripes. A perforated electrode 20 commonly called a "shadow mask" is located in close adjacency to faceplate 16 and phosphor targets 18, and is an element used for color selection. Base 22 provides entrance means for a plurality of electrically conductive lead-in pins 24.
The electron gun 26, indicated schematically, is located within neck 12 substantially as shown. Gun 26 is commonly installed in axial alignment with a center line X--X of picture tube 10. In three-beam color picture tubes utilizing the shadow mask, gun 26 emits three electron beams 28 to selectively activate target elements 18.
Power supply 30, also shown schematically, provides voltages for operation of the cathode ray tube 10 and its associated electron gun 26. A special voltage divider circuit is typically incorporated into the power supply to provide a range of potentials required for operation. For example, power supply 30 may supply relatively low voltages in the 1-8 kilovolt range through one or more conductors represented schematically by lead 32, which enters the envelope of tube 10 through one of a plurality of lead-in pins 24 in base 22. Power supply 30 also supplies selected intermediate voltages to the focus electrodes of electron gun 26, voltages typically in the range of 8-15 kilovolts or higher; these voltages are indicated as being supplied to the electrodes within the envelope of tube 12 by way of lead-in pins 24 through conductor 33. The relatively high voltage for electron gun operation, that is, a voltage typically in the range of 25 to 35 kilovolts for excitation of the accelerating anode, is indirectly supplied to gun 26 through lead 34, which is connected to anode button 36. Anode button 36 in turn introduces the high voltage through the glass envelope of funnel 14, making internal contact with a thin, electrically conductive coating 38 disposed on the inner surface of funnel 14, and extending part-way into neck 12. The anode electrode of gun 26 receives the relatively high anode voltage through a plurality of metallic gun centering springs 40 extending from gun 26, and in physical contact with inner conductive coating 38.
An electron beam diverging from the cross-over of electron gun defines a "half-angle" with respect to the axis of the gun. The half-angle is essentially a measure of beam growth in diameter as the beam diverges from the cross-over. A half-angle and means for its measure are shown schematically by FIG. 2. A cathode 35 is shown as emitting a stream of electrons which is formed into a beam 37. A cross-over 39 is formed from which beam 37 diverges. The slope of expansion of beam 37 defines an angle .alpha. with respect to the axis X--X of the electron gun. Angle .alpha. is measured from a selected "cut line" 41, which is, essentially, a point where the equipotential lines are perpendicular to the axis X--X. The half-angle is given good approximation by the expression EQU .alpha.=(0.22I.sup.1/3 /V.sub.b) radians,
for a neutral accelerating prefocus, where I is beam current in microamperes, and V.sub.b is beam voltage in kilovolts.
It is known that electron guns of particular types; e.g., bipotential, unipotential, guns having extended field lenses etc., have an optimum half-angle providing optimum spot size for the type. Further, each individual gun may have its own particular optimum half-angle that provides best performance in terms of the smallest possible spot size within the inherent capability of the particular gun.
The three principal contributors to the size of the focused beam spot on the screen are magnified cross-over, spherical aberration and space charge repulsion in the field-free space between the gun and screen of the picture tube. FIG. 3 shows graphically the dependence of spot size versus .alpha. for a gun with fixed P, Q, V.sub.a, I.sub.max and W, where P is the distance from the image to the lens center, Q is the distance from the lens center to the beam spot, I.sub.max is four milliamperes (representing beam current), and V.sub.a is the ultor anode potential. "W" is a factor that defines the quality of the lower end of an electron gun according to the formula ##EQU1## where V.sub.x is the voltage of the first electrode of the main focus lens ("G3"), and d.sub.x is the diameter of the cross-over.
FIG. 3 also shows each of the three separate contributors to spot size. The optimum spot size is indicated as being on the nadir of resultant curve 47. FIG. 3 shows that for fixed parameters there is only one half-angle from the crossover that gives the minimum spot size on the screen. FIG. 4 shows optimization curves for different W's. It can be seen, in general, that for a practical range of W (10-60), the optimum half angle stays approximately the same. In other words, the most important parameter in the design is the magnitude of the emerging half-angle .alpha., typically measured in milliradians.
A complete treatment of these and other key aspects in gun design is presented in a journal article titled "Theoretical and Practical Aspects of Electron Gun Design for Color Picture Tubes", by I. M. Wilson. (IEEE Transactions on Consumer Electronics, Volume CE-21, No. 1, February 1975.)
The establishment of the most effective half-angles in electron guns has been based almost entirely on the electromechanical design parameters of the lower end sections. The factors that affect formation of the crossover and subsequent prefocusing of the beam prior to its entry into the main focus lens and the resultant half-angle, include the configuration of the first and second grids, spacing between the cathode and first grid, and between the first and second grids and the following element of the main focus lens, aperture sizes, and the configurations of the grids as designed to establish the prefocusing fields. Once these factors are established, being of mechanical structure they are relatively fixed and unchangeable by any influences external to the envelope of the cathode ray tube. So heretofore it has not been feasible by means of external adjustments to achieve the optimum half-angle for any particular electron gun.
U.S. Pat. No. 4,009,410 to Pommier et al discloses an electron gun comprising at least one supplementary electrode of the "diaphragm" type arranged between the accelerator grid and the anode of the gun, and placed at a positive potential lower than that of the anode. The supplementary electrode, or diaphragm, is said to constitute, in association with the anode aperture, a second electrostatic condenser lens to produce a second cross-over in the electron beam. The anode voltage is varied to achieve the proper efficiency for each of the three beams producing three colors. The benefits are alleged to be better image definition, a constant cathode current and constant modulation, and brilliance modulation as a linear function of the cathode current.
Schwartz in U.S. Pat. No. 4,095,138 discloses an electron gun characterized by having at least one arc-inhibiting electrode disposed between the grid means of the tetrode and the initial end electrode of an extended field main focus lens. the arc-inhibiting electrode has a potential thereon that is intermediate to the disparate potentials of the grid means and the initial end electrode to provide an arc-inhibiting voltage gradient between the grid means and the initial end electrode between which may exist widely disparate potentials with a potential difference in the range of tens of kilovolts.